Race Distribution

These are percentages describing what percent of the continent’s dig sites are of that culture. A 25% next to Troll at Northrend would mean that 25% of the dig sites on that continent are Troll. A higher percent indicates a higher chance of getting that dig site on that continent, so that is obviously more desirable and is helpful information if you are hunting a specific faction’s rare. Source

Nerubian
In addition to upping the number of Nerubian sites in Northrend slightly, Blizzard added two Nerubian sites to Eastern Plaguelands, requiring 375 Archaeology, of course. I’m not going to create a new column in the table just for 375-450 Archaeologists, so you’ll just have to live with the numbers being here!
Eastern Kingdoms (post-375, pre-450): 3.92% (2/51)
Eastern Kingdoms (post-450): 3.64% (2/55)
Northrend: 24.14% (7/29)Verdict: Northrend

Vrykul
Only one possible area for these.
Northrend: 44.83% (13/29)Verdict: Northrend

Tol’vir
Only one possible area for these as well, but luckily, Patch 4.0.6 has nearly doubled the amount of Tol’vir sites in Kalimdor.
Kalimdor (post-450): 23.19% (16/69)Verdict: Kalimdor (post-450)

Tol’vir and Nerubian are the two rarest races to get things from. Luckily, there are only two Nerubian artifacts of any value, so you won’t have worry about them very often.

Also, the information above is based on assumption, like most things in the world. This assumption is that the the number of dig sites of each culture is directly proportional to the chance the dig site is selected. However, this assumption appears to be accurate though. The results appear to justify the assumption (low Tol’vir/Nerubian rates, etc).